The invention relates to a method for the production of a hollow body which is subjected to a torque and/or an axial force during its production while being internally supported, for the purpose of impressing internal stresses.
Various methods and arrangements are known which aim at the introduction of internal stresses into hollow bodies, composed of fiber-reinforced layers of plastics, during the production thereof. The internal stresses increase the fatigue strength of the bodies during their utilization. Such hollow bodies, which may be utilized, for example, as drive shafts in automobiles, offer, in comparison with metal constructions, the advantage of lower weight and decreased sound transmission. However, they suffer from the risk of cracks forming, due to forces or stresses in the layers acting perpendicular to the alignment of the fibers, which decrease the fatigue resistance of the hollow body during operation. Such crack formation is counteracted by the introduction of internal stresses.
In order to generate internal stresses, it is necessary to subject the hollow body during its production to forces and torques, e.g., while being heated. In order to prevent a decrease in the diameter of the hollow body during the application of such loads, the body must be supported from the inside. The use of a mandrel towards such end is known. However, during the generation of the internal stresses, i.e., during the application of the outside forces and torques, such a mandrel suffers from the disadvantage that it obstructs deformations, at least of the inner layer of the hollow body, in the direction of the circumference and in the axial direction.
With respect to this aspect, some known methods and arrangements which produce the inner supporting force by pneumatic or hydraulic pressure are of greater advantage. However, the necessity of sealing of these arrangements creates problems. This is frequently the reason for breakdowns in the production process.